1. Field of the Invention
The present invention relates to a cooling fan, and more particularly relates to a sleeve bearing of the cooling fan having good lubricating characteristics.
2. Description of Related Art
With the continuing development of the electronic technology, electronic packages such as CPUs (central processing units) are generating more and more heat that is required immediate dissipation. Cooling fans are commonly used in combination with heat sinks for cooling CPUs. Cooling fan performance mostly depends on performance characteristics of bearings used. Good lubricating qualities of the bearings increase the life-span of the bearings.
A conventional cooling fan includes a rotor having a stainless rotary shaft extending downwardly from a central portion thereof, a sleeve bearing defining an inner hole receiving the shaft therein, and a frame forming a central tube at a middle portion thereof receiving the bearing therein. The sleeve bearing is made of sintered metal powders and has a plurality of pores impregnated with lubricant oil. However, the cooling fan is usually arranged to be upended, and thus the rotor is arranged under the frame. An open end of the central tube is thus located at the bottom of the central tube and opened downwards. The lubricant oil impregnated in the bearing is thus easily to flow out the bearing due to gravity. In addition, since the amount of pores in the sleeve bearing is limited by the sintering process, the amount of the lubricant oil which can be impregnated in the bearing is also limited. Thus, after a period of time of use of the cooling fan, the lubricant oil in the sleeve bearing flows out the sleeve bearing through the open bottom end of the central tube under the influence of gravity and is lost; the lubricant oil of the sleeve bearing is thus gradually diminished. Accordingly, after a period of time, the rotary shaft and the sleeve bearing will experience wear due to the leakage of the lubricant oil contained in the bearing. As a result, the performance of the cooling fan is deteriorated, and the life-span thereof is shortened.
Therefore, it is desirable to provide a cooling fan wherein one or more of the foregoing disadvantages may be overcome or at least alleviated.